JPS6236082A - Porous ceramic structure - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention mainly relates to a porous material that is effectively used as a collection carrier for collecting fine particles floating in exhaust gas discharged from an internal combustion engine or as a catalyst carrier for exhaust gas purification. quality ceramic structures.

[Conventional technology]

Conventionally, as this type of porous ceramic structure, for example, a porous ceramic structure consisting of a three-dimensional mesh ceramic skeleton having an internal communication space is known as having excellent air permeability and purification properties.

In this type of porous ceramic structure,
It is known that a non-porous ceramic coating layer is formed on the outer periphery in order to prevent the passing gas from blowing out in a direction other than the downstream direction and to increase mechanical strength. Japanese Utility Model Application Publication No. 54-96748 discloses that by making the thermal expansion coefficient of this ceramic coating layer 0.5 to 1.5X10-''/'C smaller than that of the porous ceramic, Disclosed is a material that does not cause talac or peeling even when subjected to impact and thermal stress.

[Problem that the invention seeks to solve]

However, in the above-mentioned conventional porous ceramic structure having a non-porous ceramic coating layer, if the coating layer is thickened in order to increase the strength, for example, after forming this structure, a three-dimensional network skeleton is formed. In the process of forming an activated γ-alumina layer on the surface to increase the surface area or supporting an exhaust gas purification catalyst, there is a temperature difference between the outer periphery of the reinforcing layer and the inside of the reinforcing layer during reheating. There is a problem in that uniformity occurs, and non-uniform thermal expansion occurs within the reinforcing layer, making it easy for cracks to occur.

[Means for solving problems]

Therefore, in order to solve the above problems, the present invention provides a porous ceramic structure in which a reinforcing layer is provided on the outer periphery of a porous ceramic part, the reinforcing layer has a multilayer structure, and the coefficient of thermal expansion of the innermost layer is A porous ceramic structure made of a ceramic whose coefficient of thermal expansion is equal to or lower than that of the porous ceramic portion and whose coefficient of thermal expansion is smaller toward the outer peripheral layer is employed.

[Function] According to the above means, even if a temperature difference occurs because, for example, the ceramic structure is placed in a high-temperature furnace and the outer peripheral part of the reinforcing layer is heated quickly and the inside of the reinforcing layer is heated later. Since the coefficient of thermal expansion at the outer periphery of the reinforcing layer is smaller than that inside the reinforcing layer, non-uniformity in thermal expansion is alleviated. Conversely, even if a temperature difference occurs because the ceramic structure is rapidly cooled, the outer periphery of the reinforcing layer is quickly cooled, and the inside of the reinforcing layer is cooled later, the unevenness of thermal expansion is similarly alleviated. It acts like this.

〔Effect of the invention〕

Therefore, according to the present invention, it is possible to provide a highly durable porous ceramic structure having a reinforcing layer that does not cause cranking even when subjected to sudden thermal shock.

〔Example〕

The present invention will be described below with reference to embodiments shown in the drawings.

FIG. 1 (al is a front view from the exhaust gas inlet side of a porous ceramic filter 1 that collects carbon particles discharged from a diesel engine, which is an embodiment of the porous ceramic structure of the present invention; Figure (b) is a cross-sectional view in the axial direction.The ceramic filter 1 has a cylindrical shape with an outer diameter of 107 mm and a length of 801 m, and is made of cordierite ceramic. ,5izNn.

Various ceramic materials such as Al2O3 and β-spodumene may be used. At the outermost periphery, a densely structured reinforcing layer 2 is formed over a thickness of 2 mm. The interior 3 consists of a porous ceramic structure whose microstructure is illustrated in FIG.

That is, it has a skeleton 4 having a three-dimensional network structure and a ventilation section 5 surrounded by the skeleton 4. Inside the ceramic filter 1, there are 325 internal hollow holes 6 with a square cross section arranged in a grid pattern parallel to the axis across the partition wall 3a.
There are several. Further, every other hollow hole opens at the opposite end surface 7 or 8, and the other end is sealed at portions 7a and 8a. That is, there are 164 inlet holes 61 and 164 outlet holes 62 on the exhaust gas inlet side end face 7 and the outlet side end face 8, respectively.
There are 161 and 161 open zeros. The aperture ratio, that is, the ratio of the total cross-sectional area of the internal hollow holes to the entire cross-sectional area of the ceramic filter 1 is about 30%.

Next, the structure of the reinforcing layer 2 formed at the outermost periphery of the ceramic filter 1 will be explained based on the partially enlarged view of FIG. 3. The reinforcing layer 2 consists of an outer layer 2a on the outer peripheral surface side and an inner layer 2b on the inside thereof, and the outer layer 2a is made of magnesia (MgO).
It is a cordierite ceramic consisting of 13 wt% of alumina (CAlzOs), 38 wt% of alumina (CAlzOs), and 49 wt% of silica (Sift), and its thermal expansion coefficient is 1.7 x 10-
'/'C and the thickness is 111. The inner layer 2b is MgO
14wt%, A1Z O:l 37wt%, Sin, 4
It is a cordierite ceramic consisting of 9 wt%, and its thermal expansion coefficient is 1.9 MgO15iyt%, /1g 0
．． 36 t%.

It consists of S i Oz 49wt% and has a thermal expansion coefficient of 2.
It is 0x10-6/°C.

Next, a method for manufacturing this ceramic filter 1 will be explained.

-i To obtain a ceramic filter having a three-dimensional network structure, an organic foam such as polyurethane foam having a similar three-dimensional network structure is used as an aggregate, and a ceramic material is fixed to the surface of this aggregate. When this is fired, the organic compound that is the base material burns and scatters, and the surrounding ceramic material takes advantage of the fact that it has a similar structure to the base material.

That is, to mold the urethane foam, a wax-based mold release agent is applied in advance to the inner surface of the cavity of a mold having a cavity of the same shape as the ceramic filter 1 by spraying or brushing after the mold is heated above its melting point. Next, the temperature of the mold is adjusted to 30° C. to 50° C., and a urethane foam raw material mixture containing an organic isocyanate, a polyol, a foam stabilizer, a blowing agent, and a catalyst is poured into the mold container while stirring and mixing. Here, the batter isocyanate is tolylene diisocyanate, methylene diisocyanate, or a mixture of both, and the polyol is a polymer polyol consisting of a polyether polyol or a polyester polyol.
or a mixture of this and a polyether polyol; the foaming agent is water, a halogen-substituted aliphatic hydrocarbon foaming agent (fluorocarbons such as trichloromonofluoromethane), or a mixture of the two; the foam stabilizer is: Alcohol-modified silicone foam stabilizers, the catalysts include tertiary amines and organic acid salts thereof, which are effectively used as catalysts for the reaction between alcohol and isocyanate as catalysts that promote the resin formation reaction, and catalysts that promote the foaming reaction. Morpholine, ethanolamine, etc., which are effectively used as catalysts for the reaction between water and isocyanate, were used. After foaming the urethane foam raw material mixture in the cavity,
Heat cure at ~120°C for 20-60 minutes.

Next, a method for obtaining a porous ceramic filter by impregnating the urethane foam molded body with a ceramic slurry and then firing the polyurethane will be described in detail. The raw material for the ceramic slurry used for impregnation is Mg
O15t%, A1Al20z36%, SiO□49w
A mixed powder having a cordierite composition of t%, or a synthetic cordierite powder obtained by heating the above mixed powder to make a cordierite ceramic and pulverizing it, or a mixture of both with a binder such as methyl cellulose or polyvinyl alcohol, and water added. After removing the thin film present in the vents of the urethane foam by known explosion treatment, NaOH treatment, ozone treatment, etc., the urethane foam is impregnated into this slurry, and then excess slurry is removed using an air gun or centrifugal separator. death,
Dry in a drying oven at 100-150°C for 2-3 hours.

The above operations from impregnation to drying are repeated two or three times to deposit the required amount of ceramic slurry on the surface of the urethane foam skeleton.

After that, first MgO14wt%, AI! z 0337
A ceramic slurry was prepared from cordierite powder consisting of -1% and Sing 49wt% in the same manner as described above, and the N2b in the reinforcing layer was uniformly coated with a brush to a thickness of about 11A1 after firing. After being coated and formed, it was dried. Next, the outer layer 2a is made of 13 wt% MgO,
A ceramic slurry prepared from cordierite powder consisting of 0.338 wt% Alt and 0.49 wt% Si was coated on the inner layer 2b in a similar manner to a thickness of about 1 mm, and then dried.

Next, this structure was heated and sintered at 1300 to 1470°C for about 3 hours.

Next, the ceramic ivy filter 1 manufactured by the above method
The results of a heat resistance property test will be explained. Table 1 is a table showing the results of this heat resistance property test, in which the reinforcing layer 2 is composed of one layer and has the same thermal expansion coefficient as the internal porous ceramic (Comparative Example 1), and the reinforcing layer 2 is composed of one layer. By changing the composition of cordierite, the reinforcing layer 2
The coefficient of thermal expansion of 0.4 for the internal porous ceramic
×10-'/''C (Comparative Example 2), the reinforcing layer was formed with two layers, and the inner porous ceramic layer and the reinforcing layer inner layer were each 2.0.1.95.1. 90
X10-b/'C (Comparative Example 3) and 2.0.1°9.
The following two characteristics were investigated for four samples of 1.5 x 10-b/''C (Comparative Example 4).

In other words, the thermal shock resistance was visually examined to see how the reinforcing layer 2 would crack when the ceramic ivy filter 1 was sufficiently heated in a 600°C furnace and then taken out into the air and allowed to cool naturally at room temperature, and the thermal shock resistance at 600°C. This is a test result of static thermal characteristics in which the occurrence of cranking in the reinforcing layer 2 was visually examined when the reinforcing layer 2 was heated to 750° C., which is 150° C. higher, and slowly cooled in a furnace.

As is clear from the margin table below, in the thermal shock resistance test, in the one-layer structure (Comparative Example 1), the outside of the reinforcing layer 2 was rapidly cooled due to rapid heat dissipation, whereas the inside of the reinforcing layer was cooled. Due to this delay, thermal expansion becomes uneven, tensile stress is generated on the outside, and cranking occurs significantly. Even with a two-layer structure, the difference in thermal expansion coefficient between the inner and outer layers is 0.05 x 10-6/
In Comparative Example 3, which is small at about 'C', some cracks occur due to the same cause. On the other hand, the difference in thermal expansion coefficient is 0 due to the two-layer structure.
．． 2°0, 4 x 10-6/”C (this example,
In Comparative Example 4) and Comparative Example 2, even if the heat dissipation was uneven as described above, since the outer layer had a composition with a smaller thermal expansion coefficient, stress was relaxed and no cranking occurred at all. However, the difference in thermal expansion coefficient between Comparative Example 2 and Comparative Example 4 is 0°4 x 10-6/
℃, and although it is strong against thermal shock, there is a large difference in the coefficient of thermal expansion, so even if it is cooled slowly, the temperature difference is large, so when it is cooled to around room temperature, the stress strain becomes large, and a crank occurs. . From the above results, it was found that the effective difference in thermal expansion coefficient between the inner layer and the outer layer is 0.1 to 0.3 x 10-'/''c.

The reinforcing layer 2 of the porous ceramic structure of the present invention may be composed of three or more layers, and the coefficient of thermal expansion increases from the outer layer to the inner layer, and the innermost layer is the same as the inner porous ceramic or 0. The coefficient of thermal expansion is 1 to 0.2 x 10-6/°C small, and the difference in coefficient of thermal expansion between the adjacent outer layer and inner layer is also 0.
．． It is particularly effective to set it to 1 to 0.2 x 10-b/°C.

Furthermore, the porous ceramic structure of the present invention can be of a honeycomb form type in which a large number of inlet holes and outlet holes are provided in a porous ceramic having a three-dimensional network structure as in the above-mentioned embodiments, as well as in a honeycomb form type structure having no inlet holes or outlet holes. Needless to say, it can also be applied to a foam type, a honeycomb type having a large number of lattice-like passages, etc.

The reinforcement N2 can be formed by applying it by brushing, or by forming a sheet of ceramic slurry to a predetermined thickness in advance, wrapping it around the sheet, and then sintering it. Also, as a method to control the coefficient of thermal expansion,
A ceramic slurry with varying blending ratios of plate-shaped talc particles, which is the raw material for cordierite, is extruded through slits with a constant gap to orient it, and the phase of the thermal expansion coefficient in the crystal lattice axis direction of the plate-shaped talc particles is A method of reducing the coefficient of thermal expansion depending on the difference can also be effectively applied. Furthermore, it goes without saying that the multilayer structure may be formed of different ceramic materials that satisfy the range of thermal expansion coefficients specified in the present invention.

Further, the thickness of the reinforcing layer 2 of the present invention is particularly effective when it is 2n or more, and it is possible to prevent the occurrence of cranking due to uneven temperature inside the reinforcing layer due to thermal shock.

[Brief explanation of drawings]

1(a) and 1(b) are a front view and an axial cross-sectional view of the ceramic filter 1 of the present invention, FIG. 2 is an enlarged schematic diagram illustrating the fine structure of the porous ceramic, and FIG.
The figure is a partially enlarged view illustrating the structure of the reinforcement N2 of the ceramic filter 1. 1... Ceramic filter, 2... Reinforcement layer,
2a...outer layer, 2b...inner layer.

Claims (2)

[Claims] (1) In a porous ceramic structure in which a reinforcing layer is provided on the outer periphery of a porous ceramic part, the reinforcing layer has a multilayer structure, and the coefficient of thermal expansion of the innermost layer is less than or equal to the coefficient of thermal expansion of the porous ceramic part. 1. A porous ceramic structure characterized in that the outer peripheral layer is made of ceramic having a smaller coefficient of thermal expansion. (2) The reinforcing layer of the multilayer structure has a difference in thermal expansion coefficient between the adjacent outer layer and inner layer of 0.1 to 0.2 x 10^-
The porous ceramic structure according to claim 1, characterized in that the porous ceramic structure has a temperature in the range of ^6/°C.